Despite the success of visual interface builders, user interfaces are still too difficult to develop. Model-based approaches are promising but have a high threshold of use. In this paper, we describe Goliath, an easy-to-use Model Based User Interface Development Environment (MB-UIDE) with an extensible architecture. The models of Goliath are simple enough to be used by developers who currently use visual interface builders. However, these models are rich enough to better support the links between the interface and the functional core of the application than current MB-UIDEs.